Enhanced recovery methods are used to extract viscous crude oil from petroleum reservoirs located below the ground. Typically, when a reservoir is tapped by boreholes, only a small portion of the oil can be extracted using conventional methods. These methods include utilizing the natural pressures of the reservoirs to bring the oil to the surface and pumping the oil out of the reservoirs lacking such natural pressures.
Enhanced recovery methods reduce the viscosity of the crude oil in the reservoir, allowing it to be transported to the surface up through the boreholes. One such enhanced recovery technique utilizes heat and hot gases, which are used to liquify the oil in the reservoir. The heat and hot gases are generated by combustion systems located either on the surface or inside of a borehole.
Considerable attention has been given in the last decade or so to the development of combustion systems inserted into wellbores to achieve the generation of heat and hot gases near a petroleum reservoir face. Combustion located near the intended delivery point reduces the problems of wellbore heat loss and casing expansion which would be encountered with surface combustion followed by hot fluid delivery through tubing strings. In addition, better heat economy is achieved due to the reduced heat loss to adjacent non-producing bedrock.
The design of downhole gas generators involves many constraints which would not be encountered in surface combustion. Combustor diameter and length must be kept to a minimum. The financial incentive to use standard well casings of from 75/8 inches to 105/8 inches leads to torpedo-shaped combustor configurations which utilize length more than cross-section to achieve the required combustion volume. Field operating experience indicates that the total length of the equipment required for packing, combustion, and coolant addition must be within the range of 5 to 20 feet. Representative heat delivery requirements range from 10 to 75 million Btu/hr, which means that high-performance combustor design is required to achieve a very high density of heat release using turbulent combustion. Due to the resulting high velocity of injectants, some type of flame holder is required to prevent flameout after ignition is completed. The equipment must be easily insertable into a well of 2000 feet or more depth. This requirement dictates that the numbers of supply tubes, control tubes, and sensor and control wires be kept to an absolute minimum.
The equipment must be capable of reliable operation for a period of one year or more without removal for servicing. During this period a multiplicity of ignitions and shutdowns must be accommodated, and re-ignition may have to be performed in the presence of exiting wellbore pressures as high as 2000 psia. It is the problem of ignition of a downhole gas generator, without removing the generator from the borehole, to which the present invention is directed.
In Hamrick, et al., U.S. Pat. No. 3,982,591, there is described a downhole gas generator burning hydrogen and oxygen in a one-stage cylindrical combustor. One embodiment discloses an electrical ignition means using a glow plug or spark plug which is activated by a downhole ignition transformer which is in turn powered by wires from the surface. Although this approach would be operable, it has the disadvantage of requiring additional wires in the wellbore. Furthermore, the severe environment which could include high temperatures, volatilized hydrocarbons, steam, and salts near the combustor would accelerate deterioration of these downhole wires through degradation of or leakage through insulation. The current art of wellbore cables is such that a service life of over one year would be unlikely, severely degrading the reliability of such an ignition system.
Another embodiment of U.S. Pat. No. 3,982,591 discloses an electrical ignition means using a glow plug or spark plug which is activated by a downhole battery. An electrical contact is coupled to a downhole valve to connect the battery for ignition, and a heat switch is coupled to the generator wall to detect successful ignition and turn off the ignitor. Since the downhole valve is already required to control the injection of combusting and cooling fluids, this ignition approach answers the concern to minimize the number of downhole wires. However, it has the disadvantage of requiring a heat switch to be located as close to the flame as possible to minimize response time. Such a heat switch would be a mechanical device with a relatively low expected mean-time-to-failure. Also, the reliance on mechanical contacts could eventually produce intermittent operation leading to a catastrophic detonation when unburned injectants accumulate. This approach has a further disadvantage in requiring a non-rechargeable battery having sufficiently high energy to perform many ignitions as well as having a long shelf life in the severe environment previously mentioned. The current art of batteries is such that a service life of over one year would be unlikely.
In Wyatt, U.S. Pat. No. 4,463,803, there is described a downhole gas generator burning fuel and air in a one-stage cylindrical combustor. A spark plug and downhole ignition transformer are employed as ignition means. Wyatt provides an improvement over prior art by enclosing the transformer and borehole wiring in a metal casing to protect them from the severe environment. Nevertheless, this approach still requires an additional wellbore conduit to support the ignition means. The extra conduit could be eliminated by running the wires inside a supply tube and using high-pressure wire feedthroughs (such as are manufactured by Conax Corporation, Buffalo, N.Y.) to convey the wires out of the tube at both ends. However, this assembly would be very difficult to make and service in the field.
A further drawback of the glow plug or spark plug is their short lifetime in this highly reactive combustion environment. Sandia National Laboratories was not able to achieve more than a four-week lifetime for glow plugs in either an air-diesel or oxygen-diesel burner.
An ignition means based on autocatalytic ignition of a flowing fuel and oxygen stream through a noble metal catalyst bed would have the advantage of utilizing the same supply tubes as are used to sustain combustion. The U.S. space program has had success in using such a catalyst to ignite hydrogen and oxygen in rocket steering motors. In Berry, et al., U.S. Pat. No. 3,712,375, there is described an open tube combustor for heat generation which is ignited and sustained by a catalyst. Berry teaches that the fuel must contain at least 10% by volume of hydrogen or be preceded by a slug of hydrogen to avoid the need to preheat it. In the presence of a platinum catalyst, hydrogen will react with air at temperatures as low as 20 degrees F. and with oxygen at even lower temperatures. All other fuels surveyed required injectant preheating to at least 200 degrees F. Although other fuels could be used to sustain combustion after being initiated by a slug of hydrogen, reliability would be enhanced only if the catalytic action is sustained to serve as a fail-safe flame holder. Furthermore, additional valving would be required to divert other fuels around the catalyst. Therefore this ignition means can only be effectively applied to gas generators utilizing hydrogen fuel. Other means must be sought for generators using cheaper alternative fuels which are more suited to some steam delivery applications.
An ignition means based on bringing together a hypergolic fuel and oxidant combination has appeal because methods can be devised to use the combustor's fuel and oxidant supply tubes and to ignite any fuel-oxidant combination desired. Several liquid and gaseous pyrophoric compounds have been widely applied in the art since 1960 for initiation of in situ combustion by autoignition with air in an open fashion at the bottom of a well. Due to their inherent simplicity of use, pyrophorics and other hypergolic combinations have been considered as ignition means for downhole gas generators as well. The earliest reference is Hamrick, et al., U.S. Pat. No. 4,050,515, which mentions the use of a hypergolic combination of fuel and oxidizer to effect ignition of a downhole hydrogen-oxygen combustor, but does not define a process.
Hamrick, et al., U.S. Pat. No. 4,053,015, further defines various hypergolic combinations, an ignition sequence, and associated hardware to perform this sequence. In the described process, a slug of hypergolic fuel is introduced into the generator fuel supply line and allowed to descend by gravity and pool behind the downhole valve while it is closed. A slug of oxidizer or gaseous oxidant pressure is likewise introduced into the generator oxidant supply line. Normal generator fuel and oxidant pressures are established behind the slugs, and the downhole valves are opened to start ignition. Although Hamrick and Rose claim any process wherein the starter fuel is different from the generator fuel, they teach that the starter fuel is preferably a liquid. This recommendation follows from the reliance upon gravity to properly position the starter compounds so that their flows can start simultaneously. In the preferred embodiment, a number of hypergolic combinations are listed, and the fuel component in each of these is liquid at bedrock temperatures (except lithium borohydride, which would have to be diluted by a solvent since it is a solid). Of the listed choices, triethylborane (TEB) or triethylaluminum (TEA) together with air or oxygen are the best combinations. Some of the other choices generate highly corrosive byproducts, and some represent severe safety problems regarding toxicity or explosive instability.
Liquid TEB has been described in prior art as an ignition means for downhole gas generators which burn liquid fuels (for example, diesel and crude oil). In Wagner, et al., U.S. Pat. No. 4,336,839, there is described a downhole generator utilizing a liquid fuel atomizer feeding into a small air mixing chamber followed by a larger combustion chamber. The preferred embodiment uses a hypergolic slug such as TEA/TEB conveyed by a separate supply line. A "U" tube is described as feeding into a downhole storage tank for the pyrophoric, but design details and a sequence of operation are not described or illustrated. In Retallick, U.S. Pat. No. 4,397,356, there is described a catalytically-enhanced generator. A hypergolic fuel preceding the gaseous or liquid hydrocarbon fuel is mentioned as a possible ignition means. Similarly, Retallick, U.S. Pat. No. 4,445,570, mentions but does not detail the use of a hypergolic fuel.
Several downhole gas generators have been successfully designed and tested which burn liquid fuels with air and which utilize liquid TEB for ignition. Sandia National Laboratories achieved reliable ignition of both air-diesel and oxygen-diesel burners by inserting a slug of TEB ahead of the normal fuel and thus utilizing one supply line and atomizer means for both ignition and combustion phases. The generator detailed by Burrill, Jr. et al., U.S. Pat. No. 4,456,068, and tested by Sandia utilized the same ignition means.
Our experience has shown that problems arise when a liquid pyrophoric slug is used in a downhole generator which burns a gaseous fuel. We designed and built a hydrogen-oxygen burner which included a multiplicity of separate stages in order to achieve a large turndown range as described in Rose, et al., U.S. Pat. No. 4,199,024. A pilot stage was employed as a low-velocity mixing chamber for pyrophoric ignition followed by fuel-rich hydrogen-oxygen combustion at a moderate temperature to serve as a flame holder for succeeding stages. To minimize the number of conduits, a liquid TEB slug was used in the hydrogen line as taught by Hamrick and Rose and described previously. Tests with a volume of from 0.5 to 3.0 cubic inches of TEB led to rapid destruction of the pilot exit nozzle and the oxygen injector.
This was because of the vast difference in flow characteristics between the liquid slug and gaseous hydrogen. The orifices of the fuel and oxygen injectors had been sized to project the hydrogen-oxygen flame toward the center of the pilot chamber while holding injector pressure drops within workable limits. A nozzle which operated in critical flow with a nominal 1500 psig supply had been installed directly upstream of each downhole valve to set flow independently of chamber pressure, and the downhole valve in turn fed through a 12-inch long tube to the pilot injector. When the fuel and oxygen valves were opened, the liquid slug was forced through the injector at a velocity of 500 ft/sec. The time to inject a 0.5 cubic inch slug was a very short 27 milliseconds, whereas the time to flow enough oxygen to consume it was 8.5 seconds. Consequently, the TEB was splattered onto the chamber wall and other components due to the high velocity and lack of oxygen. Then the incoming oxygen reacted with these splatters at nearly the adiabatic flame temperature (estimated to be 5500 degrees F.) and started a steel fire. If the flow control nozzle and downhole valve were interchanged so that the liquid flow would be metered, then an unacceptably long delay of 21 seconds would ensue while the fuel tube filled behind the injector. In conclusion, the design parameters for a good hydrogen injector do not make a good liquid injector.
The ideal solution to this problem is to insert a volume of a gaseous pyrophoric compound ahead of the hydrogen so that the ignitor flow characteristics more nearly match those of the fuel. We rejected phosphine (PH.sub.3) due to its extreme toxicity. Tests with silane (SiH.sub.4), either pure or mixed with hydrogen, have shown that it is not pyrophoric at ambient temperature if the pressure is above 230 psia. Trimethylborane (TMB) was rejected since relatively little is known about it and it is only produced in small quantities by special order. The solution to our problem was the use of TEB in a vaporized state and diluted with hydrogen.